Bug report - wrong castling in chess

[sotetsuk]

I have come across what appears to be a bug and would like to report it. In the following FEN position (Left Fig.), the castling on the kingside, which should not be possible for the black player, is incorrectly deemed as possible by the legal_actions() function. Furthermore, when attempting to perform this castling, one of the rooks mysteriously disappears (Right Fig.).

Buggy state: 4k1rr/1b1p3p/nn1p4/P3Np2/3P1bp1/6PP/P5R1/1B1K2N1 b k - 1 37

You can find the steps to reproduce this issue in the following gist.

https://gist.github.com/sotetsuk/77f72626a2d89c6be24adcd44b6c7a8f

Buggy state	Wrong king side castling by black

[lanctot]

@matthewlai can you take a look?

[matthewlai]

This is due to broken partial support for chess960. I will investigate more tomorrow.

[lanctot]

This is due to broken partial support for chess960. I will investigate more tomorrow.

@matthewlai has a fix (thanks!), and we'll update the master branch as soon as it's sufficiently tested.